1. There's no exact frequency it becomes non-directional. It will also vary for different people and installs. You'll just have to try different crossover points in your install. Some SQ based installs might be crossed over around 60Hz, SPL/SQL installs will be higher, 70-100+Hz.
2. I assume you mean front and sub channels? Some frequencies will play audibly in both. The steeper your crossover slope, the less overlap you will have. Typical slopes are 12, 18, and 24db / octave. It means that at one octave away from your crossover point the sound will be reduced approximately that many decibels, another octave further it will be reduced again by that amount and so on.